In the manufacture of integrated circuits and semiconductor devices, semiconductor substrates such as wafers are moved through many processing operations or steps by using wafer transfer robots. These processing operations may also include washing and cleaning. To achieve high throughput, multiple robots may be used in a continuous in-line process. Semiconductor wafers are stored in wafer cassettes for convenience in handling the wafers in groups. Broken or missing wafers may cause further damages to other wafers in a same cassette or different cassettes. It is highly desirable that the robots are capable of handling the semiconductor wafers accurately, reliably and without damaging the surface of the wafer, and a missing or broken wafer should be detected in time to prevent further damage to other wafers.